This invention relates to a method of treatment of gas purification plant washing liquor, effluent liquor or other liquor so as to remove therefrom sodium thiosulphate.
It is known from, for example, our British patent specification No. 948,270 to practice removal of hydrogen sulphide from fuel gases by washing the latter with an aqueous alkaline solution of anthraquinone disulphonic acid and inter alia an alkali metal vanadate. The principal, and desired, chemical reaction occurring is the aerial oxidation of hydrogen sulphide to elemental sulphur catalyzed by the two active ingredients of the solution: EQU H.sub.2 S + 1/2O.sub.2 .fwdarw. H.sub.2 O + S (1)
however, some side reaction oxidation of hydrogen sulphide to thiosulphate ion occurs to an extent depending on such factors as the solution pH, solution temperature and the oxygen content of the gas to be purified. EQU e.g., 2HS + 1/2O.sub.2 .fwdarw. S.sub.2 O.sub.3.sup.= + 4H.sup.+( 2)
such side reaction oxidation is an undesirable feature of the process for several reasons, amongst which is the fact that, ultimately, the thiosulphate content of the washing liquor reaches a level at which "salting-out" losses of the active chemical ingredients, anthraquinone disulphonic acid and vanadate, is likely to occur, necessitating expensive make-up with these chemicals. Since, therefore, some thiosulphate formation is inevitable it is necessary to employ a means of removal of this compound from the wash liquor, allowing of the return of the latter to the purification plant.
It is also common practice, in the Stretford and similar processes, to recover elemental sulphur from the system by heating a sulphur slurry, derived by froth-flotation, from the main circulating liquor, in an autoclave heated to a sufficiently high temperature to cause fusion of the sulphur which may then be run off from the autoclave base. Unfortunately, the hot alkaline liquor causes a sulphur solubilization, additional to that produced by the mechanism discussed earlier, represented by the equation: EQU 2S + 6OH.sup. - .fwdarw. S.sub.2 O.sub.3.sup.= + 3H.sub.2 O (3)
thus the treated aqueous product from the autoclave contains particularly high quantities of sodium thiosulphate.
There are thus two effluent streams from a Stretford Process plant, both containing the relatively expensive anthraquinone disulphonic acid and an alkali metal vanadate, from which it is advantageous to remove sodium thiosulphate allowing for the re-use of the streams.
The process according to the invention meets this requirement and consists essentially of exposing the liquor to be treated, or a solid-residue derived therefrom by evaporation, to a hydrogen-rich gas at an elevated temperature.
In accordance with the present invention there is provided a process for the decomposition of thiosulphate ions present in aqueous gas purification liquors to sulphide and hydroxyl ions which process comprises exposing said liquor, or a solid residue derived therefrom by evaporation, to a hydrogen-rich gas at elevated temperature.
The invention also provides a process for the absorption and subsequent removal as sulphur of hydrogen sulphide from gases, gas mixture and liquid hydrocarbons, in which the material containing hydrogen sulphide is washed with an aqueous alkaline solution containing one or more anthraquinone disulphonic acids, a compound of a metal having at least two valency states, and if necessary a chelating or sequestering agent for retaining such metal compounds in solution, removing sulphur from the aqueous wash solution by froth flotation and subjecting the thus formed sulphur containing froth and/or at least part of the liquor from which the sulphur has been removed to a decomposition process as described herein.
The temperature at which the decomposition may be carried out ranges from 500.degree. to 800.degree. C, preferably from 600.degree. to 650.degree. C.
The hydrogen-containing gas may be town's gas, the reducing atmosphere existing in a flame burning with a reduced oxygen or air supply, or gaseous products obtained by the partial combustion of a hydrocarbon fuel, such as tar or heavy fuel oil.